CVE-2026-49448 authentik: SourceStage bypass via empty POST

authentik is an open-source identity provider. Prior to versions 2025.12.6, 2026.2.4, and 2026.5.1, the Source stage can be bypassed by sending an empty POST. This issue has been patched in versions 2025.12.6, 2026.2.4, and 2026.5.1...